1. Field of the Invention
The invention relates to an up-drawing continuous casting apparatus and an up-drawing continuous casting method.
2. Description of Related Art
Japanese Patent Application Publication No. 2012-61518 (JP 2012-61518 A) proposes a free casting method as a groundbreaking up-drawing continuous casting method that does not require a mold. As described in JP 2012-61518 A, a starter is first dipped into the surface of molten metal (a molten metal surface), and then when the starter is drawn up, molten metal is also drawn up following the starter by surface tension and the surface film of the molten metal. Here, a casting that has a desired sectional shape is able to be continuously cast by drawing up the molten metal through a shape determining member arranged near the molten metal surface, and cooling the drawn up molten metal.
With a normal continuous casting method, the sectional shape and the shape in the longitudinal direction are both determined by a mold. In particular, with a continuous casting method, the solidified metal (i.e., the casting) must pass through the mold, so the cast casting takes on a shape that extends linearly in the longitudinal direction. In contrast, the shape determining member in the free casting method determines only the sectional shape of the casting. The shape in the longitudinal direction is not determined. Therefore, castings of various shapes in the longitudinal direction are able to be obtained by drawing the starter up while moving the starter (or the shape determining member) in a horizontal direction. For example, JP 2012-61518 A describes a hollow casting (i.e., a pipe) formed in a zigzag shape or a helical shape, not a linear shape in the longitudinal direction.
The inventors discovered the problem described below. With the free casting method described in JP 2012-61518 A, the molten metal drawn up through the shape determining member is cooled and solidified by cooling gas, so a solidification interface is positioned above the shape determining member. The position of this solidification interface directly affects the dimensional accuracy and surface quality of the casting. Therefore, it is essential to detect the solidification interface and control it to within a predetermined reference range.
Here, the inventors have found that, because the surface of the drawn-up molten metal oscillates (more specifically, greatly fluctuates in short fluctuation cycles) and the surface of the casting formed by the molten metal solidifying does not oscillate much at all (more specifically, fluctuates little in long fluctuation cycles), the solidification interface can be determined based on whether there is oscillation. However, if the position of the solidification interface is low, oscillation of the drawn-up molten metal is small and is difficult to detect, so it is difficult to determine the solidification interface based on whether there is oscillation. As a result, if the position of the solidification interface is low, the solidification interface may not be able to be controlled to within an appropriate reference range.